Circuit for starting electric motor from standstill with maximum torque

ABSTRACT

The wheels of a vehicle are driven by synchronous motors energized from cycloconverters controlled by signals of selectively variable magnitude and phase and modulated at motor frequency. The signals for controlling the cycloconverters are generated in the output winding of a rotary vector adder having a pair of ninety-degree displaced energizing windings inductively linked to the output winding. Maximum motor torque is available at start and the displacement angle of the motor voltage is kept to zero and motor voltage is adjusted to a magnitude which will limit motor current to a value not exceeding 125 percent of full load by a current regulated starting circuit having a first operational amplifier which compares a current feedback signal proportional to motor current to a selectively variable power signal proportional to the desired output torque from the motor and set by a foot pedal, a current control amplifier which subtracts a speed signal proportional to motor r.p.m. from the power signal to derive a current control signal which sets maximum motor current, and a variable limit clamping emitter follower which receives the output from the first operational amplifier and provides an output voltage to only one energizing winding of the rotary vector adder and clamps the magnitude of said output voltage to an upper limit which varies with the current control signal so that the voltage applied to the vector adder energizing winding decreases as motor speed increases until the starting circuit output becomes zero at a predetermined motor speed.

United States Patent 1191 Ringland et al.

1 Oct. 29, 1974 I CIRCUIT FOR STARTING ELECTRIC Primary Examiner-Gene Z. Rubinson MOTOR FROM STANDSTILL WITH Attorney, Agent, or Fi'rmLee H. Kaiser MAXIMUM TORQUE 1751 Inventors: William L. Ringland, Greendale; I 1 ABSTRACT Manfred Nellmalm w Berlin; The wheels of a vehicle are driven by synchronous Ems! f 'i west Allis; motors energized from cycloconverters controlled by Thomas GllmOfe, Wauwatosa; signals of selectively variable magnitude and phase Allols F. GelerSbaC M wa e all and modulated at motor frequency The signals for f controlling the cycloconverters are generated in the [73] Assignee: Allis Chalmers Manufacturing Output wmdmg of F f i f gi 7 Company9 Milwaukee wisof nmety-degree dlsp aced energ zing win ings in ne- I twely linked to the output winding. Maximum motor Flledi J 1973 torque is available at start and the displacement angle '21 Appl No 326,612 of the motor voltage is kept to zeroand motor voltage ad usted to a magnitude which Wlll limit motor cur- Relaled pplleatlon Data rent to a value not exceeding 125 percent of full load [601 Division ofScrt Not91.115,Nov, 19, 1970, ,which is by a urr nt r gulated starting r uit a ng a fi it division 01 Scr. No. 851462, Aug 27, 1969, operational amplifier which compares a current feed- Whwh IS 11 commualiondn-parl 0f 324,223, back signal proportional to motor current to a selec- MHY I969- Pat 3534276- tively variable power signal proportional to the desired output torque from the motor and set by a foot pedal, lszl u 318/398 318/400 318/410 a current control amplifier which subtracts a speed I'll- Cl. 1 i t t i i ignal roportional to mote from the power l58l F'eld Search 3l8/394- 3981400 signal to derive a current control signal which sets 318/410, 635 maximum motor current, and a variable limit clamping emitter follower which receives the output from [5m References cued the first operational amplifier and provides an output UNITED STATES PATENTS voltage to only one energizing winding of the rotary 3,201 .675 11/1965 cumin et al 318/635 X vector adder and Clamps the magnitude of Said Output 3,3 I 7,806 5/1967 Farinelli et al... 318/410 voltage to an upper limit which varies with the current 3378.743 4/1968 Weiser 1 1 4 1 1 .1 318/400 control signal so that the voltage applied to the vector $510,737 5/1970 Brown ct al. 1. 318/635 X adder energizing winding decreases as motor speed in- 2 229 creases until the starting circuit output becomes zero 5.. anal eta. l

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SHEET 0'4 0F 14 PAIENTEMN 29 1914 saw us or 14 PATENIEBMI 29 I074 sum as or 14 wig v MO K PATENTEDUBT 29 M4 SHEET PATENTEBBBI 29 I874 saw 13 or 14 CIRCUIT FOR STARTING ELECTRIC MOTOR FROM STANDSTILL WITH MAXIMUM TORQUE This is a division, of application Ser. No. 9i ,l filed Nov. l9, I970 which is a division of our application Ser. No. 853,462 filed Aug. 27, 1969 which is a continuation-in-part of our application Ser. No. 824,223 filed May l3, I969, now US. Pat. No. 3,584,276.

Electrically driven vehicles may employ a diesel, gasoline, or turbine engine as the primary source of power, or prime mover. The prime mover may drive one or more generators which, in turn, power one or more electric motors connected to the wheels or tracks of the vehicle. On wheeled vehicles it is often advantageous to utilize a separate motor for each wheel. An electric control may be interposed between the engine-driven generators and the motors of such a vehicle to regulate the application of electrical power to the motors and wheels.

In the past, most electrically driven vehicles have utilized direct current electrical apparatus. Direct current motors, particularly those of the series types, are readily adaptable to electric drive systems because of their ease ofcontrol, and further the electrical controls for direct current motors and generators are simple and well developed However, the commutators and rotating armature coils required by direct current machines add to their manufacturing and maintenance cost.

Electrically driven vehicles are known which employ alternating current motors that are simple and reliable in construction. However, since the speed of an alternating current motor is determined by the frequency of the alternating current source, a control must be provided to convert the fixed frequency of the source to the required motor frequency which varies with speed.

The simplest type of alternating current motor is the induction motor, but the differential, or slip, between the speed of the rotor and that of the rotating stator field presents difficulties in using a frequency detector driven by the motor shaft for controlling the frequency converter which supplies the variable frequency to the motor stator. The stator frequency must be higher than the detected rotor frequency by the amount of the slip frequency. and further the slip results in heat losses in the rotor which are difficult to remove, particularly at low speeds. The torque of an induction motor is proportional to the square of the voltage-to-frequency ratio applied to the stator winding. For constant power and constant rpm slip conditions, the voltage applied to an induction motor over a desired speed range must increase as the square root of the applied frequency. In a typical instance of lo to 1 frequency (and motor speed) range at constant power, the maximum supplied voltage required is four times the minimum voltage. Inasmuch as the size of the power supply in a drive system, including the generator and power controlling elements. is determined by the maximum voltage required as well as the maximum current required, a large power supply is necessary for an electric drive system using induction motors.

Because the speed of a synchronous motor ISe inherently proportional to applied frequency. motors have been employed in constant speed drive systems where the motor is energized from a constant frequency source such as a 60 cycle per second power line. When energized at this frequency, the torque of a synchronous motor assumes a level sufficient to cause the motor to drive the load in synchronism with the rotating stator field. Such conventional synchronous motor operation is one of constant speed, variable power due to the variable motor torque.

It is an object of the invention to provide an electrically driven vehicle wherein the power applied to the tractive elements of the vehicle may be individually selected and controlled, thereby permitting steering by controlling the power and speed of the tractive elements on either side of the vehicle.

A further object of the invention is to provide a current regulated starting circuit for the synchronous motors of an electrically driven vehicle which effects maximum motor torque at start and keeps the displacement angle of the motor voltage at zero and adjusts motor voltage to a value which will keep motor load current within predetermined limits.

Another object is to provide such a current regulated starting circuit for a synchronous motor of an electrically driven vehicle which provides a terminal voltage for said motor which increases linearly from a standstill to a predetermined motor speed and also varies in magnitude in accordance with the selectively variable position of a foot pedal on the vehicle.

These and other objects and advantages of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. I is a schematic diagram in block form of the electric drive system ofa vehicle embodying the invention;

FIG. 2 is a sectional view through a vehicle wheel and the electric motor which drives it in the embodiment of FIG. 1;

FIG. 3 is an exploded perspective view of the electric motor and the angle sensor driven by it in the embodiment of FIGS. 1 and 2, the motor stator and windings being shown as encapsulated in resin;

FIG. 4 is a partial front view showing the rotor and stator of the angle sensor;

FIG. 5 is a graph plotting relative torque provided by the electric drive system versus relative speed;

FIG. 6 is a graph showing the no-load saturation curve and the rated armature current, zero power factor saturation curve for the electric motor of the embodiment of FIGS. 1-5; 7

FIG. 7a shows the simplified equivalent circuit of the electric motor; FIG. 7b shows its vector diagram for motor operation; and, FIG. 7c shows its vector diagram for generator operation;

FIG. 8 is a graph plotting motor terminal voltage and displacement angle versus speed required to provide maximum rated power over the speed range;

FIG. 9 is a development of the angle sensor stator and rotor and also schematically illustrates the instantaneous voltages generated in the angle sensor secondary windings;

FIGS. 10a and I0!) are schematic diagrams illustrat ing vector addition of the two input signals to the angle sensor primary windings to derive the output signal which controls the cycloconverter;

FIG. II is a graph plotting the motor and angle sensor voltages versus motor speed required to obtain con stant percent and I00 percent maximum power over the speed range;

FIG. 12 is a schematic diagram in block form of the angle sensor control, the discriminator, and the clip ping circuit;

FIG. I3 is a schematic circuit diagram of the function generators of the angle sensor control;

FIG. 14 is a schematic circuit diagram of the cycloconverter and filter and showing the firing circuit in block form;

FIGS. ISa through 1511 schematically illustrate voltages in the cycloconverter of the electric drive system;

FIG. 16 is a schematic circuit diagram of the tachom- BILI';

FIG. 17a is a schematic circuit diagram, partially in block form, of the firing circuit for the controlled rectifiers of the cycloconverter; and FIGS. 17b through 17] show signals at various points within the firing circuit of FIG. I7;

FIG. 18 is a simplified schematic diagram of the relay logic circuit and the motor control;

FIG. 19 is a graph plotting the output voltage VR from current control amplifier versus motor speed;

FIG. 20 is a graph plotting motor field current versus power pedal position to provide constant power over the speed range for a selected power level;

FIG. 21 shows curves plotting as polar coordinates the variation of motor terminal voltage VT and displacement angle DT with motor speed illustrated in FIG. 8;

FIG. 22 is a schematic circuit diagram of the turn compensation means; and

FIG. 23 is a schematic circuit diagram of the speed limit circuit.

GENERAL DESCRIPTION Referring to FIG. I. a vehicle embodying the invention has an electric drive system which provides mo tive power to the right front. left front. right rear and left rear wheels l2. l4, l6 and 18 of the vehicle, respectively. A prime mover on the vehicle is preferably a hydrocarbon-fueled engine such as a gas turbine or a diesel engine which operates at substantially constant speed. as determined by the setting of its governor. and is capable of providing constant power input to electric drive system [0 under all conditions of vehicle operation.

Electric drive system It) preferably includes four electric controls, or drives 22. 24, 26 and 28 (shown in block form) for the wheels l2, l4. l6 and I8 respectively. All four drives are substantially identical, and only drive 22 for right front wheel 12 will be described.

Prime mover 20 drives a first generator which provides electrical power for electric drives 22 and 24 that operate front wheels I2 and I4 and also drives a second generator 32 which provides electrical power for electric drives 26 and 28 which operate the vehicle rear wheels 16 and [8. Generators 30 and 32 are similar and may be conventional high frequency polyphase alterna tors. and only generator 30 will be described. Generator 30 preferably has a stationary exciting coil 34 energized from a suitable power supply (not shown) and three wyeconnected armature windings 36A, 36B and 36C which generate three phasc alternating current voltages A. B and C in high frequency, constant power buses 38A. 38B and 38C respectively. Prime mover 20 runs at nearly constant speed so that the input power to electric drive system 10 and the frequency of the al ternating current generated by generators 30 and 32 are nearly constant.

The input signals to electric drives 22, 24, 26 and 28 which control the transmission of power from generators 30 and 32 to vehicle wheels I2, l4, l6 and 18 are preferably derived from conventional driver-operated vehicle controlling apparatus including a steering wheel 40, a travel direction and speed limit selector 42, a power pedal 44, and a brake pedal 46 which are coupled through a master driver-operated control circuit 48 to electric drives 22, 24, 26 and 28.

Electric drive 22 includes a synchronous motor 50 which preferably is of the inductor type and has a rotor 52 mechanically coupled to right front wheel 12 through appropriate gearing (shown in FIG. 2), a threephase armature winding, or stator winding 54, and a field winding 56 mounted on the motor stator. Electric drive 22 converts the constant frequency. constant voltage output of high frequency generator 30 to a vari able frequency, variable voltage, variable phase alternating current for application to armature winding 54 to regulate the torque and speed of synchronous motor 50.

Electric drive system 10 transmits the constant power output of prime mover 20 to the vehicle wheels l2, l4, l6 and 18 over a wide range of speed. The hyperbolic torque versus speed characteristics of electric drive sys tem 10 to accomplish such transmission of constant power from prime mover 20 to the vehicle wheels over a speed range of IS to l is shown in FIG. 5. Torque and speed are conveniently related for purpose of description to base values taken at the minimum speed at which constant maximum power is transmitted. The base rating of electric drive system 10 wherein relative torque is arbitrarily designated l.t) and relative speed designated 1.0 is shown in FIG. 5 wherein the full line curve corresponds to the maximum power capability of diesel engine 20 with power pedal 44 fully depressed and with total power equally divided between the individual wheel drives 22, 24. 26 and 28. However, drive system 10 includes means described hereinafer to reduce motor torque under operating conditions requiring less than rated power as represented by the dotted line curve designated reduced power" in FIG. 5. Vehicle speed, motor speed, and motor frequency are all directly proportional because of the use of synchronous motors 50 and the fixed ratio gearing between motors 50 and wheels l2, 14, I6 and 18. Further, motor torque and vehicle tractive effort are also directly proportional because of such fixed ratio gearing.

In the preferred embodiment of the invention motor 50 is of the synchronous inductor type, although any type synchronous motor having adjustable field excita tion, including the conventional salient pole type, may be used in electric drive system [0. The conventional saturation curves of synchronous motors are commonly used to show the relationship between stator terminal voltage VT and field current I, for various load conditions. Since synchronous motors are usually operated at constant speed corresponding to a terminal voltage having a fixed frequency of 60 cycles per second, such saturation curves are usually shown for a fixed frequency. The effect of armature resistance on these synchronous motor characteristics is usually small but be' comes increasingly significant as the frequency approaches zero.

When synchronous motor 50 is operated at variable frequency, as in electric drive system 10, more meaningful saturation curves which illustrate the effects of frequency variation and armature resistance at low speeds can be derivegay re placing the terminal voltage VT by the variable (VT+ l,.R,,)/F as illustrated in FIG. 6 which shows the no-load saturation curve and the rated armature current, zero power factor saturation curve for a typical synchronous motor 50 suitable for electric drive system It) and where:

VT is the motor terminal voltage;

1,, is the armature current;

R, is the effective armature resistance; and

F is the frequency,

It will be noted that the values shown in FIG. 6 are expressed as per unit quantities relative to the base rating of the motor. 4

The vector quantity VT I,,R,, is an internal voltage. commonly called the voltage behind the resistance, and represents the vector sum of the terminal voltage and the armature resistance drop, observing the appropri ate phase angle between the two vectors. Commonly used convention considers the power component of armature current to be positive for generator operation and negative for motor operation. Therefore, for motor operation the vector V T ER is less than VT for power factors in the normal operating range near unity where W and 1,, are displaced approximately 180 (see FIG. 7b). The total quantity (VT+ I,,R,,)/F is proportional to the net magnetic flux linking the armature winding of motor 50, which net magnetic flux is the resultant of that generated by the armature winding 54 and field winding 56. j

The saturation curves of FIG. 6 permit close approximation, by well known methods, of the field current 1, corresponding to any load condition. In the preferred embodiment of the invention, electric drive 22 holds the field current I, in motor 50 constant for a desired armature current corresponding to a given position of power pedal 44. As motor speed and frequency increase above the base value F= l.0, the electric drive 22 of the preferred embodiment also holds the terminal voltage VT constant for a given power pedal position, thereby resulting in tl i net nagnetic flux, as represented by variable (VT R,,l,,)/F decreasing very nearly inversely proportional to frequency F. These conditions are represented in FIG. 6 for rated armature current by the vertical line at I 2.5 designated constant l for L 1.0." and it will be noted that the variable (V T +T,,R,.)/F decreases from a value of 1.0 at frequency F: l.0 along this constant field current line to a value of 0.5 at frequency F: .0 and then decrease further to a value of 0.2 at F 5. Below frequency F It). the terminal voltage VT is reduced by electric drive 22 as described hereinafter but at a rate that results in the net tlux rising to a maximum as the frequency approaches zero. With the field current I, and terminal VT constant, the armature current 1,, is close to rated value (1,: L0) and the power factor close to unit over the entire range of frequency, thereby resulting in the electric drive system I0 having the hyperbolic constant power speed-torque characteristic shown in FIG. 5. Assuming that electric drive system is designed to have a power rating matching that of die sel engine 20, this speed-torque curve of FIG. 5 might alternatively be designated maximum electric drive system continuous capability. When less than rated power is required to drive the load and power pedal 44 is not fully depressed, the speed-torque characteristic may be represented by the dotted line curve designated reduced power in FIG. 5.

Electric drive 22 regulates the terminal voltage VT applied to armature winding 54 of electric motor 50 as described above and also shifts the applied terminal voltage VTthrough the required phase angle relative to the angular position of the rotor to obtain the constant power torque-speed characteristic shown in FIG. 5. Such required phase angle is best described by reference to FIG. 7a, which shows the simplified equivalent circuit of synchronous motor 50 under steady state conditions, and to FIGS. 7!) and 7c which show its vector diagram for motor and generator operation respectively and wherein:

E is the internal voltage proportional to the field current;

X, is the effective synchronous reactance for all field positions relative to the armature m.m.f.; and

R is the effective armature resistance. The armature current I results from the vector voltage difference acting on the machine impedance, and thus:

1! (E!) and E): :1 I! FIG. 7b illustrates the vector diagrain of this relationship for motor operation approximating the conditions in electric drive 22 at rated load for speed F= L0, and FIG, 7c illustrates the vector diagram for generator operation approximating the conditions in electric drive 22 at speed F: 1.0, with braking power less than rated load. The displacement angle DT is the phase angle between the internal voltage 15,, and the terminal voltage VT applied to stator winding 54.

Electric drive 22 advances the terminal voltage VT in phase by the angle (+JDT relative to the physical axis of the internal voltage E,, on the rotor in order to produce the conditions for motor operation illustrated in FIG. 7b, and it delays the terminal voltage in phase by the angle ()DT relative to the physical axis of E,, in order to provide the conditions for generator action shown in FIG. 70.

In the equivalent circuit and vector diagram of FIG. 7, the internal voltage E and effective synchronous reactance X, are proportional to frequency and are multiplied by F for operation at other than base frequency F: [.0

FIG. 8 illustrates the terminal voltage VT and displacement angle DT which electric drive 22 applies to stator winding 54 of synchronous motor 50 at rated load to provide the desired constant power speedtorque characteristic of FIG. 5 with constant field current, essentially constant armature current and power factor close to unity. It will be noted that the displacement angle DT is 0 at F: 0.0 and approaches at high frequency and motor speed. FIG. 8 shows only two VT curves designated rated power and reduced power, but it will be appreciated that a different VT curve exists for each position of power pedal 44 and the corresponding power output level from motor 50.

Electric drive 22 includes a frequency changer, or cycloconverter 58 which is supplied with constant voltage, high frequency power from generator 30 over buses 38 and is responsive to gating signals from a firing circuit 60 to convert this high and constant frequency power to a lower variable frequency terminal voltage VT supplied over conductors 62 to stator phase windings 54X, 54Y and 542 of synchronous motor 50. Cycloconverter 58 is shown in detail in FIG. I4 and pref erably includes a positive group of three thyristors, or silicon controlled rectifiers associated with each of the three motor stator phase windings 54X, 54Y and 542 to carry positive current from the three-phase power busses 38A. 38B and 38C and a negative group of three silicon controlled rectifiers associated with each of these three motor stator phase windings to carry negative current from the busses 38A, 38B and 38C.

Firing circuit 60 derives gating signals which cyclically fire the silicon controlled rectifiers in cycloconverter 58 at desired points in the cycles of the high frequency. constant magnitude voltages A, B and C in busses 38A, 38B and 38C to generate the three-phase voltages in conductors LI, L2 and L3 which are ap plied to stator phase windings 54X. 54Y and S4Z.

Electric drive 22 includes a rotary inductor. vector adder, or resolver. termed "angle sensor" 64 driven by motor 50 for deriving a control signal for cycloconverter 58 which is a replica in magnitude, frequency and phase ofthe voltage VT(shwn in FIG. 8) to be applied to stator winding 54 to obtain constant power over the speed range. In order to keep the poles generated in motor rotor 52 locked in with the revolving poles generated by motor stator winding 54, the frequency of the terminal voltage VT applied to stator winding 5| must at all times be in synchronism with rotor speed. and further the terminal voltage must be advanced in phase at all motor speeds by the displacement angle DT between the magnetic flux produced by the field current acting alone and the magnetic flux corresponding to the terminal voltage. Further. the magnitude ofthe terminal voltage VT impressed on stator winding 54 must be controlled as a function of motor speed in the manner shown in FIG. 8.

The variation in magnitude VT and phase angle DT ol' the terminal voltage shown in FIG. 8 to be applied to stator winding 54 can be expressed graphically by the loci of an equation in which motor speed is the variable parameter and the magnitude VT and phase angle D1 are the radius vector and vectorial polar coordinates of the curve formed by the loci. FIG. 21 shows such a curve in dotted lines designated rated power plotting the loci of terminal voltage VTand phase angle DT when motor 50 is delivering rated power and also shows a curve in full lines designated reduced power." The magnitude of the terminal voltage VT to be applied to stator winding 54 as a function of motor speed is the radius vector ofsuch curve, two such vectors VT] and V72 for the reduced power curve being shown in FIG. 2I. The displacement angle by which the terminal voltage is to be advanced in phase relative to the rotor poles as a function of motor speed is the vectorial angle of the curve. two vectorial angles D71 and D72 for the reduced power curve being shown. It will be noted that the magnitude ofthe terminal voltage VT is maintained constant from the base speed F: l.() at the lower limit ofthe speed range (shown by vector VT] having phase angle D'II J with increase in motor speed. while the displacement angle increases from approximately 40 at F I It) to approximately 90 at speed F= 3.5. at which the terminal voltage is shown by the radius vector V12 and the displacement angle by the vectorial angle DTZ.

A plurality of different curves can be plotted in FIG. 21 all of which are ofthe same shape and each of which represents a different position of power pedal 44 and a corresponding different power output level from motor 50. Each curve shown in FIG. 21 can also be defined by its rectangular coordinates .i' and y which vary as a function of motor speed F. or by the parametric equations of the curve having motor speed F as the variable parameter. Electric drive 22 includes an angle sensor control 76 which generates a pair of sine" and cosine" signals V, and V. representative of the y and x rectangular coordinates of a curve of FIG. 21 for each position of power pedal 44, and angle sensor 64 vectorially adds such signals and derives an output signal for controlling cycloconverter 58 whose magnitude and phase are in accordance with the radius vector and vectorial angle polar coordinates of such curve. Inasmuch as a curve of FIG. 21 is the loci of an equation which expresses the desired variation in magnitude VT and phase angle DT with motor speed as shown in FIG. 8, the terminal voltage VT applied by cycloconverter 58 to motor stator winding 54 is in accordance with one of the curves of FIG. 8 corresponding to a given power pedal position. As explained in detail hereinafter, the output signals from angle sensor 64 regulate firing circuit 60 which derives the gating signals for firing the thyristors of cycloconverter 58.

Rotary inductor, vector adder. or angle sensor 64 in effect converts the signals V and V representative of the y and .r rectangular coordinates of a curve of FIG. 2! into the polar coordinates of such curve. Angle sensor 64 has a secondary winding 66 comprising three Wye-connected secondary phase windings 66X. 66V and 662 displaced l20 (electrical) and a pair of energizing, or primary windings, termed sine winding 68 and cosine winding 70, displaced 90 degrees electrically from each other magnetically coupled with secondary winding 66. The primary windings 68 and 70 and the three phase secondary windings 66X, 66Y and 662 are wound on an angle sensor stator 72 (see FIGS. 2. 3 and 4) mounted on the housing of motor 50, and the magnetic flux linkage between the secondary winding 66 and the primary windings 68 and 70 depends on the air gap between the stator 72 and a ferromagnetic rotor 74 connected to the motor rotor 52. The angle sensor rotor 74 is contoured to produce an approximately sinusoidal variation in the air gap and in the flux linkage between each secondary phase winding 66X, 66Y and 662 and the primary windings 68 and 70 as it rotates.

The sine and cosine primary windings 68 and 70 are separately excited with high frequency. in-phase sine and cosine signals V and I. from angle sensor control 76 which are in accordance with the y and x rectangular coordinates respectively ofa curve of FIG. 21 corresponding to a given position of power pedal 44. If motor 50 is at standstill and the angle sensor control 76 were to energize the sine and cosine windings with fixed magnitude. in-phase, high frequency alternating signals, the displaced sine and cosine windings 68 and 70 would have constant ampere turns and induce fixed magnitude. high frequency signals in the three phase secondary windings 66X. 66Y and 66Z. The permeances of the paths for the magnetic flux generated by the sine and cosine windings 68 and 70 and the voltage level of the fixed amplitude signals induced in the three-phase secondary windings is a function of the po sition of angle sensor rotor 74. When motor 50 rotates, the high frequency angle sensor output voltages induced in secondary windings 66X, 66Y and 662 are no longer fixed in amplitude but rather have a sinusoidal modulation at a relatively low frequency which is representative of the speed of the motor 50. The envelopes of the three angle sensor output signals induced in the secondary phase windings 66X, 66Y and 662 are displaced 120 degrees (electrical) because of the physical location of these windings on the angle sensor stator 72.

The term output voltage" is used herein to connote either the carrier frequency signals induced in secondary phase windings 66X, 66Y and 662, their low frequency sinusoidal modulation envelopes, or the signals derived therefrom by demodulation to remove the carrier frequency and alternate half waves. The carrier frequency signals induced in secondary phase windings 66X, 66Y and 662 are designated w, and their low frequency modulation envelopes and the signals derived therefrom by demodulation are designated VT,,. Further, all three such forms of output voltage" are directly proportional in magnitude, and their magnitude is represented in curves by the designation VT,

Angle sensor control 76 receives a reference power signal from protection and regulation circuit 78 (see FIG. I) which is a function of the position of power pedal 44 and independently controls the signals V, and V to sine and cosine windings 68 and 70 of angle sensor 64 as a function of this power signal, thereby controlling the ampere turns of these windings and the magnitude of the signals v1, (and their modulation envelopes VT.) induced in secondary phase windings 66X, 66Y and 662 which, after demodulationi control cycloconverter 58. Thus angle sensor control 76 suitably regulates the magnitude of the signals V, and V to the sine and cosine windings 68 and 70 of the angle sensor 64 to control the magnitude of terminal voltage VT applied to the motor stator winding 54 as a function of power pedal position, Change in position of power pedal 44 varies the magnitude of the power signal and thus changes the length of the radius vector VT in FIG. 21.

Angle sensor control 76 also receives a speed" signal from tachometer 80 (see FIG. 1) which is a function of the speed of the motor 50 and modifies the signals V, and V,. to the sine and cosine windings 68 and 70 in response to the speed signal in accordance with the y and .r rectangular coordinates of a curve of FIG. 2| so that the angle sensor output voltages VT, induced in secondary windings 66X, 66Y and 662 vary linearly (see FIGS. 8 and ll) from zero at zero speed to full value (corresponding to a given position of power pedal 44) at base frequency F= 1.0 and remain at full value from F 1.0 to maximum motor speed F= l5.0. Above base speed F= 1.0, angle sensor control 76 unequally varies the magnitude of the signals V, and V, to the sine and cosine windings 68 and 70 in opposite di rections as a function of motor speed (see FIG. 11) while maintaining the magnitude of the angle sensor output voltage VT, constant for a given power pedal position, thereby unequally varying the ampere turns and the intensity of the magnetic flux which these windings 68 and 70 generate and shifting the phase ofthe modulation envelopes VT, of the angle sensor output voltages induced in secondary windings 66X, 66Y and 66Z relative to the angular position of the angle sensor rotor 74 while maintaining the magnitude of these voltages constant. The output signals VT, from angle sensor secondary winding 66 control cycloconverter 58, and angle sensor control 76 thus regulates the displacement angle DT of the voltage VT applied to motor stator winding 54 relative to the magnetic poles on motor rotor 52 as a function of motor speed, while maintaining the magnitude of the terminal voltage VT constant for a given position of power pedal 44, by independently varying the magnitude of the sine and cosine signals V, and V to sine and cosine windings 68 and to obtain the terminal voltage versus speed and displacement angle versus speed characteristics designated VT and DT in FIG. 8.

The three-phase carrier frequency output voltages vt, from angle sensor secondary windings 66X, 66Y and 662 are demodulated in a descriminator 82 to remove the carrier signal. Discriminator 82 provides threephase output voltages VT, of motor frequency controlled in magnitude and phase angle relative to a reference axis or the motor rotor, i.e., the axis of internal motor voltage E shown in the vector diagram of FIG. 7b.

The peak voltages of the low frequency output signals VT, from discriminator 82 are limited in a clipping circuit 84 which derives flat-topped output signals VT, (see FIG. 12) when the voltages from the discriminator 82 exceed the clipping level.

When the speed of motor 50 can be regulated over a speed range wherein the motor frequency voltages VT, applied to stator winding 54 vary from zero to f Hertz, the generator 30 provides voltages A, B and C having a frequency of at least Zfand preferably 3fwhen diesel 20 is controlled by its governor to run at top speed.

In order to condition the generator high frequency, constant magnitude voltages A, B and C in the busses 38A, 38B and 38C for generating the synchronizing, or sequence" signals required by firing circuit 60, a filter 86 (see FIG. I) removes the commutation notches and high frequency noise from the generator voltages A, B and C and regulates the voltage level thereof to derive reference" voltages which are reproductions of the fundamental waves of these generator voltages. Firing circuit 60 combines these three-phase, high frequency output reference" signals from filter 86 with the three-phase low frequency control signals VT, from clipping circuit 84 (which are proportional to the angle sensor output voltages VT to derive the "sequence signals A+VT B-l-VT, and C+VT, shown in FIG. 15c, and the firing circuit 60 includes level detectors (described hereinafter) which sense the zero crossing points of such sequence signals and generate the gating signals for the controlled rectifiers of the cycloconverter 58.

As explained hereinbefore, the terminal voltage VT applied to stator winding 54 and displacement angles DT shown in FIG. 8 will result in the desired speedtorque characteristic of FIG, 5 with constant field current. essentially constant armature current, and power factor close to unity. Voltage drops occur in generator 30 because of the commutating inductance, and additional resistance and reactance voltage drops occur in the controlled rectifiers and center tapped reactors of cycloconverter 58, and the effect of such voltage drops 

1. A circuit for starting an electric motor from standstill with maximum torque and limiting motor current during starting comprising, in combination, means for deriving a selectively variabale power signal whose magnitude is a function of the desired output torque for said motor, variable limit clamping means for clamping the magnitude of said power signal to an upper limit which varies in accordance with the magnitude of a signal applied to a control input thereof, tachometer means for deriving a speed signal which is a function of the speed of said motor, means for subtracting said speed signal from said power signal to derive a current control signal which varies inversely with the speed of said motor, said current control signal being applied ot said control input, means for deriving a current feedback signal which is a function of the current supplied to said motor, and means for comparing said current feedback signal to said power signal, the output from said comparing means being an input to said variable limit clamping means.
 2. A circuit in accordance with claim 1 wherein said variable limit clamping means includes an emitter follower transistor having its base coupled to the output of said comparing means and its collector coupled to the output of said subtracting means, and a diode having its cathode and anode coupled respectively to said collector and said base, said means for supplying a terminal voltage to said motor being coupled to the emitter of said emitter follower transistor.
 3. A circuit in accordance with claim 2 wherein said comparing means includes an operational amplifier which receives said power signal and said current feedback signal on its inverting and its noninverting inputs respectively.
 4. A circuit in accordance with claim 2 wherein said subtracting means includes an operational amplifier which receives said power signal and said speed signal as inputs and whose output is said current control signal and is applied to said collector of said emitter follower transistor.
 5. A circuit in accordance with claim 1 wherein said variable limit clamping means includes a diode having its anode and its cathode coupled respectively to the output of said subtracting means and to the output of said comparing means.
 6. A circuit for starting an electric motor from standstill with maximum torque and for limiting motor current during starting comprising, in combination, means for deriving a selectively variable power signal whose magnitude is a function of the desired output torque from said motor, variable limit clamping means for clamping the magnitude of said power signal to an upper limit which varies in accordance with the magnitude of a signal appearing on a control input thereof, said variable limit clamping means including an emitter follower transistor whose base receives said power signal and whose collector is said control input and a diode connected across said collector and said base, means for applying a signal to said collector which varies inversely with the speed of said motor, and means for applying A terminal voltage to said motor whose magnitude is a function of the output from said variable limit clamping circuit.
 7. A circuit for starting an electric motor from standstill with maximum torque comprising, in combination, means for deriving a selectively variable power signal whose magnitude is a function of this desired output torque from said motor, tachometer means for deriving a speed signal which is a function of the speed of said motor, means for deriving a current feedback signal whose magnitude is a function of the current supplied to said motor, means for comparing said current feedback signal to said power signal, means for subtracting said speed signal from said power signal to derive a current reference signal, variable limit clamping means coupled to the output of said comparing means for limiting the output thereof to an upper limit which varies in accordance with the magnitude of a signal applied to a control input thereof, said current reference signal being applied to said control input, and means for supplying a voltage to the terminals of said motor whose magnitude is a function of the output from said variable limit clamping means.
 8. A circuit in accordance with claim 7 wherein said means for applying a terminal voltage to said motor includes a cycloconverter having output coupled to said motor, and means including a pair of displaced energizing windings inductively linked with an output winding for vectorially adding signals applied to said energizing windings to generate a signal in said output winding for controlling said cycloconverter which is selectively variable in phase and magnitude, and wherein the output of said variable limit clamping means is coupled to only one of said energizing windings, whereby the displacement angle of the output voltage from said cycloconverter supplied to said motor is zero at standstill.
 9. A circuit for starting an electric motor with maximum torque from standstill comprising, in combination, means for deriving a selectively variable power signal which is a function of the desired output torque from said motor, means for deriving a current feedback signal which is a function of the magnitude of current supplied to said motor, first operational amplifier means for subtracting said current feedback signal from said power signal, variable limit clamping means for clamping the output from said first operational amplifier means to an upper limit which varies in accordance with the magnitude of a signal applied to a control input thereof, tachometer means for deriving a speed signal proportional to the r.p.m. of said motor, second operational amplifier means for subtracting said speed signal from said power signal to derive a current reference signal, said current reference signal being applied to said control input of said variable limit clamping means, and means for applying a terminal voltage to said motor whose magnitude is a function of the output from said variable limit clamping means.
 10. A circuit for starting an alternating current electric motor from standstill with maximum torque and for limiting motor current during starting comprising, in combination, means for deriving a selectively variable power signal whose magnitude is a function of the desired output torque from said motor, variable limit clamping means for clamping the magnitude of said power signal to an upper limit which varies in accordance with the magnitude of a signal appearing on a control input thereof, means for applying a signal to said control input which varies inversely with the speed of said motor, means including a cycloconverter having its output coupled to the motor stator winding for applying a terminal voltage to said motor whose magnitude is a function of the output from said variable limit clamping means, and means including a pair of displaced energizing windings inductively linked with an output windiNg for vectorially adding signals applied to said energizing windings to derive a signal in said output winding for controlling said cycloconverter which is selectively variable in phase and magnitude, and wherein the output of said variable limit clamping means is coupled to only one of said energizing windings, whereby the phase angle of the output voltage from said cycloconverter supplied to said motor is zero at standstill.
 11. A control for starting an electric motor from standstill with maximum torque and limiting motor current during starting and operating the motor over a speed range comprising, in combination, means (44, 430, 436) for deriving a selectively variable power signal whose magnitude is a function of the desired output torque from said motor, means (80) for deriving a speed signal which varies in accordance with the speed of said motor, means (178, 64) including a summing circuit (192) for deriving a control signal (VTs) which varies in accordance with said power signal and also as a function of said speed signal, means (58) for applying a terminal voltage (VT) to said motor which varies in accordance with said control signal (VTs), variable limit clamping means (470) in parallel to a portion of said control signal deriving means and receiving said power signal as an input and having its output coupled to said summing circuit (192) for clamping the magnitude of said power signal to an upper limit which varies in accordance with a current limit signal (VR) applied to a control input thereof, and means (CCA) for applying a current limit signal (VR) to said control input of said clamping means whose magnitude is a function of said power signal and which varies inversely with said speed signal and becomes zero at a predetermined motor speed.
 12. A control in accordance with claim 11 wherein said motor (50) is an alternating current motor, said means for applying a terminal voltage to said motor includes a frequency changer (58) regulated by said control signal (VTs), and said means (178, 64) for deriving a control signal includes means (74) coupled to the rotor (52) of said motor for modulating said control signal at motor speed.
 13. A control in accordance with claim 12 wherein said means for deriving a control signal (VTs) includes means (138, 178, SIN, COS) for selectively varying the phase of said control signal as a function of said speed signal.
 14. A control in accordance with claim 11 wherein said means for applying a current limit signal (VR) to said control input of said clamping means includes means (CCA) for subtracting said speed signal from said power signal to derive said current limit signal (VR) which is coupled to said control input.
 15. A control in accordance with claim 11 wherein said means for deriving a control signal (VTs) includes a function generator (178) for deriving a DC signal (vts'') which varies from zero at standstill in proportion to said power signal and also as a function of said speed signal, and wherein the output of said function generator (178) is an input to said summing circuit (192). 